New chemical and refining processes, and the economies of scale of such processes, will require increasing quantities of gaseous oxygen at a single location. Requirements for 15,000 tons per day, or more, of gaseous oxygen delivered at pressures of 1,250 psia or higher are anticipated for such processes. Some of these facilities requiring large quantities of gaseous oxygen will be constructed and operated at geographically remote locations.
A large air separation unit to produce gaseous oxygen requires a compressed air feed stream which is typically provided from atmospheric air by an intercooled turbocompressor driven by an electric motor. Electric motor driven turbocompressors also are utilized to compress the gaseous oxygen and other air separation process by-products.
As the required capacities for air separation units increase, electric power requirements for compressor drivers also increase. The power demand for a large air separation unit using an electrically-driven turbocompressor may exceed the capabilities of the available electric power system. At remote locations, imported electric power may be essentially unavailable. In such cases, onsite electric power generation systems may be required.
If natural gas, liquid fuel, or fuel synthesized in a chemical or refining process is available, the turbocompressor supplying air to the air separation unit may be mechanically driven by a gas turbine combustion engine. This avoids both the need for an electric power generating and transmission system, and the associated electric generating and transmission energy losses. However, the temperature of the exhaust stream from a gas turbine combustion engine operating with a simple Brayton cycle is in the range of 700.degree. F. to 1100.degree. F., and the exhaust represents a major portion of the heat generated by combustion in the gas turbine engine. Typically the expansion turbine exhaust is used to generate steam which is expanded a steam bottoming cycle to drive an electric generator or other rotating machinery.
Comprehensive reviews of integration methods for gas turbines and air separation systems are given in a paper entitled "Next-Generation Integration Concepts for Air Separation Units and Gas Turbines" by A. R. Smith et al in Transactions of the ASME, Vol. 119, April 1997, pp. 298-304 and in a presentation entitled "Future Direction of Air Separation Design for Gasification, IGCC, and Alternative Fuel Projects" by R. J. Allam et al, IChem.sup.E Conference on Gasification, Sep. 23-24 1998, Dresden, Germany.
A common mode of integration between the gas turbine and air separation units is defined as full air and nitrogen integration. In this operating mode, all air for the gas turbine combustor and the air separation unit is provided by the gas turbine air compressor which is driven by the gas turbine expander, and nitrogen from the air separation unit is utilized in the integrated system. Full air and nitrogen integration is described in representative U.S. Pat. Nos, 3,731,495, 4,224,045, 4,250,704, 4,631,915, and 5,406,786, wherein the nitrogen is introduced into the gas turbine combustor. Full air and nitrogen integration also is described in U.S. Pat. Nos. 4,019,314 and 5,317,862, and in German Patent Publication DE 195 29 681 A1, wherein the nitrogen is work expanded to provide work of compression for the air feed or to generate electric power.
The gas turbine and air separation unit can operate in an alternative mode, defined as partial air integration with full nitrogen integration, in which a portion of the air feed for the air separation unit is provided by the gas turbine compressor and the remainder is provided by a separate air compressor driven by a driver with an independent power source. Nitrogen from the air separation unit is introduced into the gas turbine combustor or is otherwise work expanded. This operating mode is described in representative U.S. Pat. Nos. 4,697,415; 4,707,994; 4,785,621; 4,962,646; 5,437,150; 5,666,823; and 5,740,673.
In another alternative, nitrogen integration is used without air integration. In this alternative, the gas turbine and air separation unit each has an independently-driven air compressor, and the nitrogen from the air separation unit is used in the gas turbine combustor. This option is described in representative U.S. Pat. Nos. 4,729,217; 5,081,845; 5,410,869; 5,421,166; 5,459,994; and 5,722,259.
U.S. Pat. No. 3,950,957 and Great Britain Patent Specification 1 455 960 describe an air separation unit integrated with a steam generation system in which a nitrogen-enriched waste stream is heated by indirect heat exchange with hot compressed air from the air separation unit feed air compressor, the heated nitrogen-enriched stream is further heated indirectly in a fired heater, and the final hot nitrogen-enriched stream is work expanded in a dedicated nitrogen expansion turbine. The work generated by this expansion turbine drives the air separation unit feed air compressor. The nitrogen expansion turbine exhaust and the combustion gases from the fired heater are introduced separately into a fired steam generator to raise steam, a portion of which may be expanded in a steam turbine to drive the air separation unit main air compressor. Optionally, the combustion gases from the fired heater are expanded in a turbine which drives a compressor to provide combustion air to a separate fired heater which heats the nitrogen-enriched stream prior to expansion.
An alternative use for high pressure nitrogen from an air separation unit integrated with a gas turbine is disclosed in U.S. Pat. No. 5,388,395 wherein the nitrogen is work expanded to operate an electric generator. The cold nitrogen exhaust from the expander is mixed with the inlet air to the gas turbine compressor thereby cooling the total compressor inlet stream. Alternatively, low pressure nitrogen from the air separation unit is chilled and saturated with water in a direct contact cooler-chiller, and the chilled, saturated nitrogen is mixed with the inlet air to the gas turbine compressor.
U.S. Pat. Nos. 5,040,370 and 5,076,837 disclose the integration of an air separation unit with high-temperature processes which uses oxygen, wherein waste heat from a process is used to heat pressurized nitrogen from the air separation unit, and the hot nitrogen is work expanded to generate electric power.
European Patent Publication EP 0 845 644 A2 describes an elevated pressure ir separation unit in which the pressurized nitrogen-enriched product is heated indirectly by the combustion of low pressure fuel, and the hot nitrogen is work expanded to produce power or drive gas compressors within the air separation unit.
As indicated the above discussion of the background art, the recovery of heat from a gas turbine combustion engine exhaust typically is achieved by a heat recovery steam generation (HRSG) system which comprises a heat exchanger with numerous boiler tubes to vaporize boiler feed water, a steam turbine for work expansion of the steam, a condenser to condense the expanded steam, and a boiler feedwater makeup and recirculation system. In some situations, however, such a steam system may not be feasible for economic or operational reasons, and alternative methods for recovering heat from the gas turbine exhaust would be required. When the preferred method for driving a turbocompressor supplying air to an air separation unit is by a gas turbine combustion engine, such an alternative method for recovering heat from the gas turbine exhaust would be desirable.
The invention disclosed below and defined by the claims which follow addresses the need for gas turbine driven air separation units which use alternative methods of recovering and utilizing the heat in the gas turbine exhaust gas.